I study how choice behavior given unawareness of an event differs from choice behavior given subjective belief of zero probability on that event. Depending on different types of unawareness the decision-maker suffers, behavior under unawareness is either incomparable with that under zero probability (in the case of pure unawareness), or drastically different (in the case of partial unawareness). The key differences are (1) partial unawareness permits dynamically inconsistent choice, while zero probability beliefs do not; and (2) there are unforeseen options in an unawareness environment that are necessarily modeled as dominated options in zero probability models.